Excimer lasers, also known as rare gas halide lasers, use a mixture of gases in a discharge chamber, excited by an external electrical power source, to produce a laser beam. A typical gas mixture is composed of (i) approximately 0.2 percent halogen; (ii) approximately 1 to 2 percent rare gas; and (iii) the balance a buffer gas such as helium, neon or a mixture thereof. The various gases are injected into the discharge chamber in the proper ratios, and the chamber is then sealed.
The gain, and thus the output of the laser, is directly related to the concentrations of the different gases in the chamber gas mixture and to the purity of the mixture. The gain is particularly sensitive to the concentrations of the halogen and the rare gas. When the laser is operating, the halogen reacts with other materials in the discharge chamber, and the halogen is thereby depleted, thus reducing the gain of the laser. The reactions in the chamber also produce gaseous and particulate contaminants which can further reduce the laser gain.
A prior approach to the problem of halogen depletion is to compensate by controlling the excitation power applied to the laser. A feedback control system maintains the laser gain at a desired level by controlling the excitation power. Thus when the laser gain begins to fall as the halogen is depleted and contaminants are formed, the control system increases the excitation power. In this way, the gain of the laser is stabilized at the desired level. This system is described in U.S. Pat. No. 4,611,270 to Klauminzer et al., entitled "Method and Means of Controlling the Output of a Pulsed Laser".
Excitation control effectively stabilizes the gain of the laser within a predetermined, and somewhat limited, dynamic range. Eventually, however, halogen depletion reaches a point where the gain cannot be maintained by further increasing the excitation power.
Another control arrangement restores the gain of the laser by injecting additional halogen into the discharge chamber. However, since the amount of additional halogen required is quite small, the halogen is typically pre-mixed with the buffer gas and this mixture is then injected into the chamber. The inspection thus not only raises the concentration of the halogen in the discharge chamber, it also raises the concentration of the buffer gas, which is not depleted during laser operation.
In order to maintain optimum total chamber pressure following the gas injection, a portion of the chamber gas can be removed. However, removing the gas from the chamber causes the amount of rare gas in the chamber mixture to be reduced below the optimum value, and thus, the gain is reduced.
A further method used to maintain adequate concentrations of the various gases in the discharge chamber is to store a mixture of all the gases, in the optimum proportions, and then use this mixture to replace a portion of the gas in the chamber. Thus when the concentration of the halogen in the chamber is depleted, a portion of the gas in the chamber is released and pre-mixed gas is injected into the chamber to recharge it. The problem with this method is that a relatively large amount of gas must be replaced to appropriately raise the halogen level, since the halogen is only a small portion (0.2%) of the pre-mixed gas.
A modification of this method, currently in use, employs two gas sources - one container providing a mixture of all three gases and the other a mixture of the halogen and the buffer gas. The halogen-buffer source is used to replenish the halogen, and the three-gas mixture is used to replace contaminated gas released from the chamber. Eventually, the chamber mixture departs materially from the optimum concentrations, resulting in lower gain.